Die casting of wrought aluminum alloys

ABSTRACT

A method of making a wrought aluminum alloy component comprises die casting the wrought aluminum alloy in a die cavity to produce a die cast component and isostatically pressing the die cast component to promote closure of internal voids therein.

FIELD OF THE INVENTION

[0001] The present invention relates to a method of making wroughtaluminum alloy components by die casting the alloy to near net shapefollowed by isostatic pressing.

BACKGROUND OF THE INVENTION

[0002] There are two types or classes of aluminum alloys; namely, alloysthat are wrought (or worked) into shape and alloys that are cast toshape. Approximately 75% of the aluminum produced in the US is inwrought product form. In the aerospace industry, nearly all of thealuminum used is in wrought form. For example, airplane wheels are madefrom precision forged wrought aluminum alloys, such wrought alloy 7050(Al-6.2% Zn-2.3% Cu-2.3% Mg where %'s are weight %). Unlike castings,wrought products do not have to account for the alloy's ability to flowinto thin mold sections. Thus, wrought products are afforded moreflexibility in chemical composition, which can be used to provideincreased alloy mechanical properties. In addition, forgings haveessentially no porosity due to the redundant working of the alloymicrostructure.

[0003] Because of the high strength-to-weight ratio of wrought aluminumalloys, they are excellent candidates for critical componentapplications in the aerospace and automotive industries. However, as aresult of their poor castablity, wrought aluminum alloys are nottraditionally cast into near net shape components. For example, wroughtaluminum alloys exhibit poor flowability of the molten alloy into thinmold sections and poor hot tear resistance evidenced by unacceptable hottear cracks in the cast product.

[0004] Manufacture of such wrought products to complex component shapesinvolves extensive fabrication costs resulting from the multiplefabricating (e.g. heating, forging and machining) operations that mustbe performed on the initial billet of wrought aluminum alloy.

[0005] There is a need for a method of making near net shape castings ofwrought aluminum alloys that are suitable for use in applications in theaerospace, automotive and other industries.

[0006] It is an object of the present invention to provide a method ofmaking wrought aluminum alloy components that satisfies this need.

SUMMARY OF THE INVENTION

[0007] The present invention provides a method for making wroughtaluminum alloy components by die casting the alloy in a die cavity atsubambient pressure to produce a near shape die cast component having anexterior surface that is amenable to subsequent isostatic pressing topromote closure of internal voids, such as for example internalmicroporosity and/or microcracking. The die cast component preferably issubsequently hot isostatically pressed to promote closure of internalvoids.

[0008] In an illustrative embodiment of the invention, a wroughtaluminum alloy is die cast under conditions to produce a near net shapedie cast component that may exhibit internal voids that are notsurface-connected to an extent that the die cast component can besubsequently hot isostatically pressed to promote closure of theinternal voids. Particular illustrative preferred die cast conditionsinclude a combination of a high vacuum during die casting, highinjection (plunger) speeds, optional heating of the dies to superambienttemperature, high intensification pressure on the initially moltenwrought aluminum alloy after it is injected into the die cavity and fora sufficient time to allow the die cast component to completely solidifyunder pressure.

[0009] The die cast component then is hot isostatically pressed (HIP'ed)under conditions to promote closure of any internal voids, providing anear net shape die cast and HIP'ed component. Hot isostatic pressingtypically is conducted in a gaseous atmosphere at a high enoughtemperature and for a long enough time to promote closure of internalvoids in the die cast component and improve mechanical properties, suchas yield strength, ultimate tensile strength and ductility, of the diecast and HIP'ed component.

[0010] Details of the present invention will become more readilyapparent from the following detailed description taken with thefollowing drawings.

DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a schematic side elevation of a vacuum die castingmachine for practicing an embodiment of the present invention with theshot sleeve, dies, and vacuum chamber shown broken away.

[0012]FIG. 2 is a plan view of one of the dies for die casting wroughtaluminum alloy (7050) specimens.

DETAILED DESCRIPTION OF THE INVENTION

[0013] For purposes of illustration and not limitation, FIG. 1 shows adie casting machine that can be used to die cast a wrought aluminumalloy under conditions to produce a near net shape die cast componentamenable to subsequent hot isostatic pressing in practice of theinvention. The invention is practiced to die cast wrought aluminumalloys of the type described in the International Alloy Designations andChemical Compositions Limits for Wrought Aluminum and Wrought AluminumAlloys, Unified North American and International Registration Records ofThe Aluminum Association Incorporated, 900 19th Street, N.W.,Washington, D.C., January, 2001. Wrought aluminum alloys are identifiedby four digit numerical designations (e.g. 2xxx, 3xxx, 5xxx, 6xxx, etc.series numbers) as opposed to casting aluminum alloys which areidentified by three digit numerical designations. Wrought aluminumalloys which can be die cast, hot isostatically pressed, andprecipitation heat treated by practice of the invention include all 2xxxalloys that include Al, Cu, Mg, 6xxx alloys that include Al, Mg, Si, and7xxx alloys that include Al, Zn, Mg, Cu.

[0014] A die casting machine that can be used to practice the inventionis described in U.S. Pat. No. 6,070,643, the teachings of which areincorporated herein by reference. The die casting machine comprises abase 10 which defines therein a reservoir 10 a for hydraulic fluid thatis used by hydraulic actuator 12 to open and close the fixed and movabledie platens 14, 16. The platen 16 is disposed for movement on stationarytie bars or rods 18. A die clamping linkage mechanism 20 is connected tothe movable die platen 16 in conventional manner to open/close themovable die 34 relative to fixed die 32 disposed on platen 14. Forexample, a conventional die casting machine available as 250 ton HPM#73-086 from HPM, Cleveland, Ohio, includes such a base 10, actuator 12,and die platens 14, 16 mounted on tie bars 18 and opened/closed by dieclamping linkage mechanism 20 in the manner described. The die castingmachine includes a gas accumulator 21 for rapid feeding of hydraulicfluid to the plunger mechanism.

[0015] The die casting machine also comprises a tubular, horizontal shotsleeve 24 that communicates to a die cavity 30 defined between the dies32, 34 disposed on the respective die platens 14, 16. The die cavity 30can be configured to form one or more cast components. The shot sleeve24 has a discharge end section 24 a that communicates with an entrancepassage or gate 36 to the one or more die cavities 30 so as that moltenwrought aluminum alloy can be pressure injected therein. The entrancepassage or gate 36 can be machined in the stationary die 32 or themovable die 34, or both.

[0016] The discharge end section 24 a of the shot sleeve 24 extendsthrough a suitable passage 24 b in the stationary platen 14 and die 32as illustrated in FIG. 1.

[0017] The shot sleeve 24 extends through die 32 into a vacuum meltingchamber 40 where the molten wrought aluminum alloy to be die cast isheated in a crucible 54 under relatively high vacuum conditions such asabout 15 to about 50 microns. The vacuum chamber 40 is defined by avacuum housing wall 42 that extends about and encompasses or surroundsthe opposite charging end section of the shot sleeve 24 receiving theplunger 27 and the shot sleeve hydraulic actuator 25. The vacuum chamber40 is evacuated by one or more conventional vacuum pumps P connected tothe chamber 40 by a conduit 40 a. The base 10 and the vacuum housingwall 42 rest on a concrete floor or other suitable support.

[0018] The chamber wall 42 is airtight sealed with the fixed platen 14by peripheral airtight seal(s) 43 located therebetween so as tosealingly enclose the shot sleeve and a pair of side-by-side stationary,horizontal shot sleeve/plunger support members 44 (one shown) extendingthrough chamber wall 42. Such shot sleeve/plunger support members areprovided on the aforementioned conventional die casting machine (250 tonHPM #73-086).

[0019] A plunger 27 is disposed in the shot sleeve 24 for movement byplunger actuator 25 and plunger connector rod 27 b between a startinjection position located to the right of a melt entry or inlet opening58 in shot sleeve 24 and an injection position proximate the dieentrance gate 36. The melt inlet opening 58 communicates to a metal(e.g. steel) melt-receiving vessel 52 mounted adjacent the fixed platen14 on the shot sleeve 24 by clamps such as screw clamps (not shown). Themelt-receiving vessel 52 is disposed beneath crucible 54 to receive acharge of molten wrought aluminum alloy therefrom for die casting. Themelt-contacting surfaces of dies 32, 34 and plunger tip 27 a typicallyare coated with a graphite spray coating.

[0020] The crucible 54 can comprise a melting crucible in which a solidcharge of the wrought aluminum alloy is melted in chamber 40. Aninduction coil 56 is provided about the crucible to heat the alloy to adesired alloy die casting temperature with some amount of superheatabove the alloy melting point. Alternately, the crucible may be a meltholding crucible adapted to receive molten wrought aluminum alloy from avacuum melting crucible (not shown) located outside the chamber 40.

[0021] If the crucible 54 is a melting crucible, it can comprise aninduction skull crucible comprising copper segments in which a charge ofsolid wrought aluminum alloy to be die cast is charged via vacuum port40 b and melted by energization of induction coils 56 disposed about thecrucible in conventional manner in the chamber 40. Known ceramic orrefractory (e.g. graphite) lined crucibles 54 also can be used inpracticing the present invention. The crucible 54 can be tilted byrotation about crucible trunnions T using a conventional hydraulic,electrical or other actuator (not shown) disposed outside the vacuumchamber 40 and connected to the crucible by suitable vacuum sealedlinkage extending from the actuator to the crucible. The crucible 54 istilted to pour the molten wrought aluminum alloy charge into themelt-receiving vessel 52, which is communicated to the shot sleeve 24via opening 58 in the shot sleeve wall. The molten charge is introducedthrough opening 58 into the shot sleeve 24 in front of the plunger tip27 a.

[0022] The plunger 27 is moved from the start injection position to aninjection position proximate the die entrance gate 36 by a conventionalhydraulic actuator 25 that, for example, is provided on theaforementioned conventional die casting machine. Typical radialclearance between the shot sleeve 24 and the plunger tip 27 a is in therange of about 0.0005 inch to 0.020 inch.

[0023] When dies 32, 34 are closed, the die cavity 30 definedtherebetween is communicated to the vacuum chamber 40 via the shotsleeve 24 and can be evacuated through the shot sleeve. In particular,the stationary die 32 typically includes a series of grooves (one shown)on its inner face that face the opposing inner face of the movable die34 when the dies are closed. The grooves encircle or extend about thedie cavity 30 as well as gate 36 and a melt discharge openingcommunicated to the gate 36 and defined by shot sleeve end 24 a. Eachgroove receives a respective resilient, reusable high temperature O-ringvacuum seal 60 for sealing in vacuum tight manner against the matingface of the movable die 34 when the dies are closed. Only one groove andseal 60 are shown for convenience. Alternately, the seal(s) 60 can bedisposed in grooves on the mating face of the movable die 34, or on themating faces of both dies 32, 34, so as to form a vacuum tight sealabout and isolating the die cavity 30, gate 36, and shot sleeve end 24 afrom the ambient air atmosphere surrounding the exterior of the dies 32,34 when closed. The vacuum seals 60 may comprises Viton material thatcan withstand temperatures as high as 400 degrees F. that may be presentwhen the die cavity 30 is filled with molten alloy.

[0024] By use of vacuum seals 60, the die cavity 30 is isolated from theambient air atmosphere when the dies 32, 34 are closed and enables thedie cavity 30 to be evacuated through the shot sleeve 24 when the vacuummelting chamber 40 is evacuated to high vacuum levels of about 15 toabout 50 microns employed in practice of the invention.

[0025] The dies 32, 34 optionally are maintained at a superambienttemperature in a preferred range of about 200 to 400 degrees F. duringdie casting, although unheated dies may be used. For example, the dies32, 34 are heated prior to injection of the molten alloy therein by oneor more conventional electrical resistance rod heating elements (notshown) received in channels in the dies, by gas flame burners or anyother conventional die heating means, or the dies can be self-heated asa result of prior injection of molten wrought aluminum alloy charge(s)in the die cavity 30. The dies 32, 34 may also be cooled by watercooling conduits (not shown) formed internally of the dies and throughwhich cooling water is circulated to control die temperature in thepreferred range. The shot sleeve 24 similarly can optionally be heatedor cooled to control shot sleeve temperature within a desired range ofabout 200 to about 400 degrees F. using similar heating and coolingdevices.

[0026] In die casting wrought aluminum alloys in practice of theinvention, the dies 32, 34 and shot sleeve 24 are made of steel or othersuitable material for purposes of illustration and not limitation, sinceother die and sleeve materials may be used.

[0027] In accordance with an embodiment of the invention, a wroughtaluminum alloy is die cast under a combination of conditions to producea near net shape die cast component that is amenable to subsequent hotisostatic pressing (HIP'ing) to promote closure of any internal voids.The die cast conditions typically produce a die cast component thatincludes internal voids, such as internal microporosity and/ormicrocracks (i.e. micro-hot tearing), that are not surface-connected inthat the internal voids do not extend to or penetrate the exteriorsurface of the die cast component. That is, the exterior surface of thewrought aluminum alloy die cast component is substantially free of voidsthat extend from the interior to the exterior of the component such thatthe die cast component can be subjected to a subsequent HIP operation topromote closure of the internal voids.

[0028] Particular die cast conditions useful for die casting wroughtaluminum alloys using the die cast machine described above and offeredfor purposes of illustrating but not limiting the invention include thecombination of 1) a high vacuum of about 15 to about 50 microns in diecavity 30 and chamber 40, 2) optional heating of the dies 32, 34 to asuperambient temperature of about 200 to 400 degrees F., 3) highinjection (plunger 27) speeds greater than 30 inch/second, such aspreferably 30 to 80 inches/second, and 4) high intensification pressurein the range of about 5,000 to about 20,000 psi on the wrought aluminumalloy after it is injected into the die cavity 30 and for a sufficienttime to allow the die cast component to completely solidify through thecomponent cross-section under such pressure in die cavity 30. Theseconditions can be selected and adjusted for a particular wroughtaluminum alloy composition to achieve a HIP'able die cast component.

[0029] The die cast component then is hot isostatically pressed (HIP'ed)under conditions to promote closure of any internal voids. The exteriorsurface of the wrought aluminum alloy die cast component issubstantially free of voids that extend from the interior to theexterior of the component such that the die cast component can besubjected to the HIP operation to promote closure of the internal voids.Hot isostatic pressing typically is conducted in an inert gas or othergas non-reactive with the wrought aluminum alloy. A typical gaseousatmosphere comprises argon. HIP'ing is conducted at a high enoughtemperature and for a long enough time to promote closure of internalvoids, such as microporosity and microcracking (i.e. micro-hot tearing),in the die cast component and improve mechanical properties, such asyield strength, ultimate tensile strength and ductility, of the die castand HIP'ed component. For purposes of illustration and not limitation,the HIP'ing step can be conducted at 850 to 1000 degrees F. and argongas pressure of 15,000 psi for 2 hours for most wrought aluminum alloydie cast components. The HIP step can be conducted using conventionalHIP'ing apparatus.

[0030] Prior to HIP'ing, the die cast component may be solution heattreated at an elevated temperature to reduce the amount of lower meltingpoint eutectic phase(s) that may be present in the die castmicrostructure and thereby homogenize the alloy microstructure andchemistry to avoid incipient melting during HIP'ing. Suitable solutionheat treatment temperatures and times are well known for a wide varietyof wrought aluminum alloys.

[0031] After HIP'ing, the wrought aluminum alloy die cast component maybe heat treated to develop desired mechanical properties. For example,the die cast and HIP'ed wrought aluminum alloy component can besubjected to a conventional solution heat treatment followed by one ormore lower temperature aging heat treatments to develop desiredmechanical properties for the particular wrought aluminum alloy used.Illustrative of such heat treatments are the well known T6 and T7 heattreatments.

[0032] The following example is offered to further illustrate but notlimit the invention.

EXAMPLE

[0033] Wrought aluminum alloy 7050 specimens were die cast using a diecast machine of the type described above where about 3 pounds of thealloy were induction melted in the crucible 54 in the vacuum chamber 40(FIG. 1) at a vacuum level of 25 microns. The alloy was heated to acasting (pour) temperature of 1175 degrees F. to provide superheatrelative to the alloy melting temperature of 1162 degrees F. The dies32, 34 were heated to about 212 degrees F. The melted alloy was pouredinto the shot sleeve and injected into the die cavity at 30inches/second plunger speed in a shot sleeve of 3 inches diameter. Theplunger 27 continued to travel in the shot sleeve to applyintensification pressure at a maximum calculated hydrostatic pressure of6,200 psi on the alloy in the die cavity for a time of 30 seconds suchthat the alloy was fully solidified through its cross-section whileunder intensification pressure. The dies then were then opened and thesolidified die casting ejected using ejector pins. The die cavity had aconfiguration shown in FIG. 2 for one die with four elongated “finger”cavities C each measuring 13 inches long by 0.6 inch diameter andejector pin holes H as shown. The ends of the “finger” cavitiescommunicated to a “fillet” cavity F which received injected moltenwrought aluminum alloy from the entrance gate. The 7050 die castspecimens produced in this manner exhibited some internal voids whichincluded internal microporosity and microcracking (i.e. micro-hottearing) due to solidification shrinkage. Hot tearing of the die castspecimens made pursuant to the invention was substantially reduced ascompared to specimens made without the combination of high vacuum andintensification pressure and was confined to the interior of the diecast specimens as opposed to specimens that were not die cast with thecombination of high vacuum and intensification pressure. That is, theinternal voids (microporosity and microcracks) of specimens madepursuant to the invention did not connect to the exterior surface of thedie casting such that the die casting could be HIP'ed in a subsequentstep to promote closure of the internal porosity and microcracks.Specimens that were die cast without the combination of high vacuum andintensification pressure exhibited internal porosity and hot tears thatcould not be healed or closed by HIP'ing.

[0034] Chemical analysis of the die cast 7050 specimens indicated thatthere was a loss of volatile alloying elements, in particular Zn and Mg,at the high vacuum level of 25 microns used in the trials. This loss ofvolatile elements from the wrought aluminum alloy can be countered byincreasing the initial concentrations of such volatile elements in thealloy charge to be melted. Alternately, the alloy can be melted inchamber 40 at a vacuum level insufficient to cause adverse loss of suchvolatile alloying elements from the charge. For example, the chamber 40can be maintained at a vacuum level of 40 microns to reduce loss of Znand Mg while the die cavity 30 can be maintained at a higher vacuumlevel (e.g. 10-25 microns). Such different vacuum levels in the chamber40 and die cavity 30 can be achieved by modifying the above-describeddie casting machine to include a separate vacuum pumping system for thedie cavity 30 to provide the higher vacuum in the die cavity and anisolation valve (not shown) located between the chamber 40 and diecavity 30 to isolate die cavity 30 from chamber 40 until the meltedalloy is introduced into the shot sleeve 24 ready to be injected intothe die cavity. The valve then is opened to permit injection of moltenalloy from the shot sleeve into the die cavity.

[0035] While the invention has been described in terms of specificembodiments thereof, it is not intended to be thereto but rather only tothe extent set forth in the following claims.

I claim:
 1. A method of making a wrought aluminum alloy component,comprising die casting the wrought aluminum alloy in a die cavity atsubambient pressure to produce a die cast component with an exteriorsurface that permits isostatic pressing of said die cast component andisostatically pressing the die cast component to promote closure ofinternal voids therein.
 2. The method of claim 1 wherein said alloy isdie cast to produce said die cast component that includes an exteriorsurface substantially free of voids that extend from an interior of saiddie cast component.
 3. The method of claim 1 wherein said alloy is diecast with a combination of conditions including a subambient pressure insaid die cavity of about 15 to about 50 microns, injection speed greaterthan about 30 inches/second, and an intensification pressure in therange of about 5,000 to about 20,000 psi on the wrought aluminum alloyafter it is injected into the die cavity.
 4. The method of claim 3including heating dies defining said die cavity in the range of about200 to about 400 degrees F.
 5. The method of claim 1 wherein said diecast component is hot isostatically pressed in a gaseous atmosphere. 6.The method of claim 1 including the further step of solution heattreating said die cast component before said isostatic pressing.
 7. Themethod of claim 1 including the further step of solution heat treatingand aging said die cast component after said isostatic pressing.
 9. Themethod of claim 1 wherein said internal voids comprise internal porosityor cracking.
 10. A method of die casting a wrought aluminum alloycomponent, comprising die casting the wrought aluminum alloy in a diecavity at subambient pressure in said die cavity of about 15 to about 50microns, injection speed greater than about 30 inches/second, and anintensification pressure in the range of about 5,000 to about 20,000 psion the wrought aluminum alloy after it is injected into the die cavity.11. The method of claim 10 including heating dies defining said diecavity in the range of about 200 to about 400 degrees F.